Field of the Invention
The invention relates to the field of electronic reproduction technology, especially to the production of film or printing plates and, in particular, flexo printing plates. The invention relates specifically to a method and a multibeam scanning device for the ablation of film or printing plates and, in particular, flexo printing plates by laser engraving with a multi-spot array, the plates being clamped onto the circumferential surface of a drum and scanned by a plurality of laser beams.
Flexo printing plates for direct laser image setting or laser engraving conventionally include a lower carrier layer made of polyester or another flexible polymer material, a central, so-called photopolymer layer containing unsaturated monomers and elastomeric binders that are crosslinked when exposed to UV light and, as a result, prevent being washed out later during development, and also an upper laser-sensitive layer that is partially removed in predefined areas, corresponding to the information to be transferred, by the laser engraving, in order, through the photopolymer layer, to produce a mask connected integrally to the printing plate. During the subsequent UV exposure of the printing plate, the mask covers those areas of the photopolymer layer at which the laser-sensitive layer has not previously been removed and, in these areas, prevents the crosslinking or curing of the photopolymer layer so that the photopolymer layer is washed out by the developer there during the subsequent development of the printing plate. The finally developed printing plate has elevated and depressed areas. The elevated areas are disposed where the laser-sensitive layer has previously been removed by the irradiation with the laser light.
In principle, however, in the future it is also conceivable to produce the elevated and depressed areas of the flexo printing plate, instead of by the partial removal of a mask and subsequent UV exposure and washing out the printing plate, by producing a printing plate from a laser-sensitive material that is removed selectively in one step by irradiation with laser light. In such a case, with the laser light used for the ablation, instead of a negative image of the information on the mask to be transferred, a positive image of the information is produced on the printing plate itself.
In the case of the flexo printing plates used at present, the laser engraving is carried out in a laser exposer, in which the printing plate is clamped onto the surface of a rotating drum and is scanned with one or more intensity-modulated laser beams to remove the laser-sensitive layer in the subsequent printing areas on the printing plate point by point in accordance with the predefined raster. As described, for example, in applicant""s German Published, Non-Prosecuted Patent Application DE 100 24 456.4, the laser processing heads of the laser exposer, which can be moved in the axial direction of the drum, conventionally include a series of fiber lasers for the simultaneous production of a plurality of laser beams, an optical switch disposed in front of the fiber exit of each fiber laser to modulate the emergent laser beam by interrupting the laser radiation or permitting it to pass in accordance with the respective pixel data of image information to be transmitted, and an optical system for imaging the fiber exit as a multi-spot array on the material to be processed.
One possible way of increasing the processing speed of the laser exposer is to increase the number of scanning points of the multi-spot array. For such a purpose, however, a larger number of fiber lasers is needed, which is to be avoided, if possible, because of the relatively high costs of these components.
In a magazine article by B. M. Rosenheck, xe2x80x9c180 mega-pixel per second optical image recordingxe2x80x9d in SPIE Vol, 299, Advances in Laser Scanning Technology (1981), an optical system for image recording is described, in which four laser beams previously separated by beam splitters and originating from a single laser are each deflected by an AOM, in which four different frequencies are applied to each AOM, to split the laser beams up into four partial beams, and in which the partial beams are subsequently deflected by a rotating polygon mirror onto the surface of a rotating drum to scan the drum surface substantially simultaneously at sixteen scanning points. However, such a system is not suitable for laser engraving because considerably higher laser powers are required. In addition, such a system has a relatively high space requirement, which makes it unsuitable for industrial use as a laser exposer.
It is accordingly an object of the invention to provide a method and multibeam scanning device for the ablation of flexo printing plates by laser engraving that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that, without increasing the number of fiber lasers, permit an increase in the number of scanning points of the multi-spot array and that can be implemented with a low space requirement.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for the ablation of a surface on a rotating drum having an axis including the steps of locating a number of laser fiber exits beside one another in an AOM array having a number of AOMs corresponding to the number of laser fiber exits, simultaneously emitting a plurality of laser beams from the laser fiber exits, dividing up each of the laser beams into at least two partial beams after the laser beams emerge from the laser fiber exits, modulating the partial beams independently of one another, imaging the laser fiber exits on the surface of the drum with an optical system, and laser engraving the surface with the multi-spot array by moving the laser fiber exits, the AOM array, and the optical system together in an axial direction of the drum while scanning the surface with the multi-spot array in a circumferential direction of the drum.
In accordance with another mode of the invention, the surface is a film or printing plates, in particular, flexo printing plates, and the laser fiber exits are YAG laser fiber exits.
In accordance with a further mode of the invention, the partial beams are in a plane perpendicular to the beam direction to produce a two-dimensional multi-spot array, which includes the points of incidence of the laser beams, preferably disposed one above another and beside one another in rows and columns, and, to achieve line connections by an appropriate time delay and deflection of the individual partial beams, for these to strike the rotating drum at the same time and for these points of incidence to lie beside one another on a straight line at equal intervals, the straight line being parallel to the direction of an axial relative movement between the drum and the laser processing head.
In accordance with an added mode of the invention, all of the partial beams are deflected in a direction of the drum such that points of incidence of the partial beams lie beside one another in a line on the surface and partly overlap adjacent points of incidence.
To divide the laser beams up into two or more partial beams, in accordance with an additional mode of the invention, a number of voltage signals with different frequencies are applied to each AOM, the number corresponding to the number of desired partial beams and their frequencies lying within a bandwidth of the AOM in which an adequate acousto-optic efficiency is ensured. The voltage signals are preferably applied to the AOM with a time offset so that the partial beams produced strike the flexo printing plate at the same time beside one another in the axial direction of the drum.
In accordance with yet another mode of the invention, the dividing step is carried out by applying voltage signals with at least two different frequencies to each AOM.
In accordance with yet a further mode of the invention, the applying voltage signals step is carried out by applying the voltage signals to the AOM with a time offset to cause the at least two partial beams to strike the surface in a line as a result of a rotation of the drum and to partly overlap the points of incidence of the partial beams.
To shorten the overall length of the multibeam scanning device, in accordance with yet an added mode of the invention, the YAG laser fiber exits are preferably aligned so as to converge in a fan shape, in each case some of the partial beams intersecting in the vicinity of an entry pupil of the optical system, which then aligns the partial beams substantially parallel and deflects them onto the flexo printing plate, ensuring substantially telecentric imaging of the fiber exit on the printing plate. The optical system preferably includes two lenses L1, L2 that form a beam expander, with which the beam diameter of the partial beam is widened in the ratio of the focal lengths f2/f1 of the two lenses L1, L2 and their angular spacing is reduced in the ratio of the focal lengths f1/f2, before they are focused on the printing plate by a lens L3 disposed behind the lenses L1 and L2.
In accordance with yet an additional mode of the invention, the optical system has an entry pupil, and the laser fiber exits are aligned to converge in a fan shape such that in each case some of the partial beams intersect in a vicinity of the entry pupil.
Because the spacing/diameter ratio of the points of incidence of the partial beams on the printing plate cannot be reduced arbitrarily, inter alia, because the mounting of the fiber exits in a holder makes certain minimum angular spacings necessary, and because the focal lengths of the optical system for shortening the overall lengths cannot be selected to be too great, in accordance with again another mode of the invention, the laser processing head is to be tilted about the optical axis of the optical system in relation to the drum to subsequently achieve line connection by an appropriate delay of the partial beams.
In accordance with again a further mode of the invention, the partial beams are aligned in a fan shape to cover at least two planes, the points of incidence of the partial beams on the surface in each plane respectively lying on an axis forming an angle xcex1 with the axial direction of the drum, where arctan xcex1=lx/2ly, lx being a center spacing of the points of incidence in a direction of the axes, and ly being a center spacing of the points of incidence in a direction perpendicular to the axes.
In accordance with again an added mode of the invention, the laser fiber exits are substantially telecentrically imaged on the surface with the optical system.
In accordance with again an additional mode of the invention, the partial beams are substantially aligned in parallel downstream of an exit pupil of the optical system with respect to a travel direction of the partial beams and the partial beams are deflected onto the surface.
In accordance with still another mode of the invention, the partial beams have an angular spacing and each have a diameter, and the diameter of the partial beams is widened and the angular spacing of the partial beams is reduced in the optical system.
With the objects of the invention in view, there is also provided a multibeam scanning device for the ablation of a surface on a drum by laser engraving with a multi-spot array, the drum being rotatable about an axis of rotation, the device including a number of laser fiber exits disposed beside one another, an AOM array having a number of AOMs corresponding to the number of the laser fiber exits, the AOM array connected to the laser fiber exits, the AOMs receiving laser beams emerging from the laser fiber exits, the AOM array dividing up each of the laser beams into at least two partial beams each modulated independently of one another, an optical system for imaging the laser fiber exits at the surface on the drum, the optical system disposed downstream of the AOM array with respect to a travel direction of the laser beams, and the laser fiber exits, the AOM array, and the optical system forming a multi-spot array and being moveable together in a direction of the axis of rotation of the drum while the surface is scanned by the AOM array in a circumferential direction of the drum.
In accordance with still a further feature of the invention, there are provided voltage signal devices connected to the AOMs and applying voltage signals having at least two different frequencies to each of the AOMs to divide up the laser beams into the at least two partial beams.
In accordance with still an added feature of the invention, the AOM array is disposed between the laser fiber exits and the entry pupil.
In accordance with still an additional feature of the invention, the AOMs have converters and an alignment of the converters corresponds to an alignment of convergent ones of the laser beams.
In accordance with a concomitant feature of the invention, the optical system has a beam expander widening the diameter of the partial beams while reducing the angular spacing of the partial beams .
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and multibeam scanning device for the ablation of flexo printing plates by laser engraving, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.